1. Field of the Invention
The present invention relates to alignment of an ophthalmologic apparatus, and more specifically to alignment using an anterior segment image.
2. Description of the Related Art
Currently, an optical tomographic imaging apparatus based on optical coherence tomography (hereinafter referred to as “OCT”) that uses multi wavelength interference can obtain a tomographic image of a sample at a high resolution. Such an optical tomographic imaging apparatus is becoming indispensible in retina clinics as an ophthalmology instrument.
With the above-described OCT apparatus, measurement light, which is low-coherence light, is radiated onto a sample, and the backscattered light from that sample can be measured at a high resolution using an interference system or an interference optical system. Further, a high resolution tomographic image can be obtained by the OCT apparatus scanning the sample with this measurement light. Consequently, a tomographic image of the retina at the fundus of a subject's eye is acquired, which is widely used in the ophthalmologic diagnosis of the retina.
On the other hand, generally, in an ophthalmologic apparatus it is important that the examination unit (mainly the optical measurement system) in the apparatus for capturing an image is accurately aligned with the subject's eye to be examined, and that the fundus tomographic image is correctly focused.
Japanese Patent Application Laid-Open No. 2010-162424 discusses an ophthalmologic apparatus that projects an alignment mark on the cornea of a subject's eye, captures an image with an image sensor by splitting the reflected light from the cornea, detects the relative position between the apparatus and the subject's eye based on the position of the split alignment mark image, and aligns the positions of the apparatus and the subject's eye.
Further, Japanese Patent Application Laid-Open No. 2001-327471 discusses a fundus detection apparatus that detects the relative position between the apparatus and a subject's eye in a plane perpendicular to the optical axis and aligns their positions by capturing an image of the anterior segment of a subject's eye, determining an intersection between two scanning lines and a pupil edge on the image, and calculating the deviation, i.e., the misalignment, between these light paths and the pupil.
In addition, in Japanese Patent Application Laid-Open No. 10-192244, an image splitter for splitting an anterior segment image of a subject's eye is arranged in an optical system for anterior segment observation. The optical system is detachably arranged between a perforated mirror and an objective lens. The perforated mirror splits the light into an illumination light path and an observation imaging light path. The operator aligns the apparatus and the subject's eye by observing the image-split anterior segment image displayed on a monitor. When capturing an image of the fundus, alignment is performed while observing a separately-provided alignment mark image, which is not in the light path. When the subject's eye and the apparatus reach a predetermined position, the image is captured.
In Japanese Patent Application Laid-Open No. 2010-162424, a unit for projecting the alignment mark onto the cornea of the subject's eye is necessary. Further, it is not uncommon for the pupil in a subject's eye to be off center with respect to the cornea, so that if alignment is performed based on the alignment mark image projected onto the cornea, the subject's eye pupil does not match the optical axis of the apparatus. This can cause the measurement light beams to be shaded by the pupil if the pupil diameter of the subject's eye is small.
Further, in Japanese Patent Application Laid-Open No. 2001-327471, although misalignment of the subject's eye pupil in the plane perpendicular to the optical axis of the apparatus can be detected, the distance between the apparatus and the subject's eye in the optical axis direction, i.e., the operational distance, cannot be detected. Consequently, a unit for detecting the operational distance has to be separately provided.
Moreover, in Japanese Patent Application Laid-Open No. 10-192244, since the anterior segment of the subject's eye cannot be observed when capturing an image of the fundus, image capturing has to be performed while confirming that unnecessary reflected light from the subject's eye has not entered into a circumferential portion of the observed fundus image. Consequently, an experienced operator is required.
Further, when observing the fundus, it is commonly known to project an alignment mark onto the cornea of the subject's eye and perform alignment while observing this mark along with the fundus image. However, even in such an apparatus, how much the imaging optical axis deviates from the subject's eye pupil when capturing an image of the fundus cannot be confirmed.
In addition, when projecting an alignment mark onto the cornea of the subject's eye and detecting the reflected light from the cornea, if the positional relationship between the apparatus and the subject's eye substantially deviates, the reflected light from the cornea cannot be detected.